A continuously operating press for producing chip and fiberboard webs is disclosed in U.S. Pat. No. 3,874,966. Such a press comprises a central press cylinder which is journalled to rotate in upright support members. A plurality of guide and pressure rollers are disposed around the periphery of the press cylinder. An endless steel band is guided around the press cylinder, which band is subjected to tensile stress. The chips or fibers to be pressed into a web are mixed with a binder and are carried on the band. The web is formed by the effect of pressure on the chips or fibers. Such pressure emanates from two sources. Firstly, there is a surface pressure between the band and the periphery of the press cylinder, which latter is heated. Secondly, there is a nip pressure in the nips between each of the pressure rollers and the periphery of the press cylinder. Each end of each pressure roller is journalled to rotate in bearings. A continuous web, generally having a thickness of from 0.8 mm to 12 mm and a width of up to 2500 mm, is produced which is subsequently cut to desired lengths.
The pressing operation takes place between the periphery of the rotating, heated pressing cylinder and the endless steel band which is subjected to a high tensile stress. For this purpose, a spread chip cake or non-woven material is conveyed into the press by the steel band and is pressed, at an elevated temperature during the time in which the band is disposed adjacent the periphery of the press cylinder.
If the thickness of the web exceeds a certain value, a web continuously produced on a press of this type will exhibit a tendency to bend. However, this is relatively unimportant for many intended uses of the web. However, in order to widen the range of use of the chip and fiberboards produced in this manner, the tendency of the web to bend has been reduced using larger diameter press cylinders. Thus, press cylinders having a diameter of 5,000 mm have been used instead of the hitherto customary diameter of 3,000 mm. The radius of curvature is therefore larger and this reduces the tendency of the manufactured chip and fiberboard webs to bend.
On the other hand, press cylinders of this type have a weight of approximately 110 tons. The journalling of the cylinder and of the pressure roller is therefore a complex and expensive process.
A greater disadvantage resides, however, in the fact that it is extremely difficult to manufacture chip or fiberboard webs having only small permitted tolerances of thickness across the entire width of the web, bearing in mind the fact that the web is often 2,200 mm or 2,500 mm wide. Due to the heavy weight of the press cylinder and the play inevitably occurring in the bearings and between the bolted-together components of the uprights as a result of their assembly, it is extremely difficult to control accurately the thickness of the web.
A non-woven material having a thickness of, for example, 8 cm is located on a conveyer belt and is initially pre-compressed to a thickness of approximately 5 cm in a pre-press before being transferred into the main press in which it is to be compressed to a thickness of, for example, 3 mm. The first stage of the pressing operation takes place in the nip between the first pressure roller and the central press cylinder. A surface pressure of up to 20 N/cm.sup.2 is built up on the pressed chip cake by the tensioned endless steel band. The chip cake is subjected to a further very high line pressure in the ensuing nip between the next pressure roller and the press cylinder.
A web pressed in this manner and having a thickness of 3 mm has a permitted thickness tolerance of only .+-.0.2 mm for it to be suitable for further processing. The attainment of this object is very difficult to achieve due to the weight of the central cylinder, often of the order of 110 tons and due to the air trapped in the bearings and between the bolted-together components of the uprights.